Hydraotes Chaos is a complex basin located between the equatorial areas of chasma, east of Valles Marineris and the Chryse basin. The interactions in time and space of several sedimentary processes dominate the geological history of the chaos. The basin formed after the middle Noachian. The outflow channels related to the chaos probably flowed, during an early stage, to the south (Hydraotes channel) and to the north (Simud and Tiu Valles). After the capture of the southern Hydraotes channel by the chasma areas, the floods reversed direction and flowed northward into Hydraotes Chaos. Deltaic and lacustrine environments formed at this stage. The main consequence of the filling of the basin was the activation of Tiu Vallis. This scenario probably occurred intermittently several times during favorable climatic conditions. During these periods the water was liquid at the surface or, at least, with a frozen top layer. The major pieces of evidence for lacustrine sedimentation are: (1) terraces, (2) multiple strandlines and raised beaches, and (3) sediment drape on the basin floor. Hydraotes Chaos was the transition from the chasma areas next to Valles Marineris and the Chryse Basin. Its geological setting suggests that the water was released to Tiu Vallis episodically (during a large span of time, upper Noachian to middle Amazonian) from lakes. ¿ 1998 American Geophysical Union